A stock_dbxref can have any number of slot-value property tags attached to it.
This is useful for storing properties related to dbxref annotations of stocks,
such as evidence codes,
and references,
and metadata,
such as create/modify dates.
This is an alternative to hardcoding a list of columns in the relational schema,
and is completely extensible.
There is a unique constraint,
stock_dbxrefprop_c1,
for the combination of stock_dbxref_id,
rank,
and type_id.
Multivalued property-value pairs must be differentiated by rank.